Recently, a lot of effort has been put in enhancing the use of energy storage media, in particular large numbers of energy storage devices in series connection. Such series connection is for example used in hybrid drive trains for buses, waste collection vehicles, fork lift trucks and electric cars.
Usually, a series connection of energy storage devices for medium and high voltage applications comprises a large number of energy storage devices. For example, since the maximum voltage of an energy storage device is limited to for example about 2.5V to 3.0V in case of a double layer capacitor, a number in the range of 20 to 25 energy storage devices need to be serially connected to form an energy storage device stack delivering a voltage of for example 60V.
A general problem of series connections of energy storing devices is that varying characteristics of each individual energy storage device due to, for example, differences in self-discharge, capacitance, internal resistance and temperature, generates an inequality between the individual energy storage devices resulting into a so-called unbalanced stack, resulting in poorly utilized energy storage device, unless a charge equalization is performed.
In order to solve the above problem, systems for balancing a series connection of energy storage devices are developed.
The most widely used systems are shunting charge of energy storage devices using a dissipative element, such as a resistor. This is a simple but very energy inefficient approach with the major drawback that balancing needs to be kept at very slow rates to alleviate relatively high heat dissipation.
An approach that is much more energy efficient and consequently can be operated at increased speed is shuttling of charge between the energy storage devices. This can be done by using intermediate energy storage elements such as capacitive or inductive elements.
For example, WO97/44877 describes a switched capacitor system for automatic battery equalization of series coupled batteries. The system includes for each pair of batteries a capacitor and a plurality of switching elements. Each of the capacitors is switched back and forth between a predetermined pair of batteries for the purpose of transferring charge and equalizing the output voltages of each of the batteries in the pair.
A major disadvantage of the above system is that the energy charge is always to be distributed through the series connection via adjacent devices. In other words, balancing large numbers of energy storage devices may take a significant amount of time, and this amount of time increases upon extending the number of energy storage devices.
In an attempt to solve the above problem, U.S. Pat. No. 6,404,165 describes a system, wherein a capacitor is connectable in parallel with at least two energy storage devices and wherein voltage monitoring means are used to monitor respective voltages of the energy storage devices and selecting two devices to be balanced to each other.
An obvious disadvantage of this prior art system is that voltage measurement for each individual energy storage device is used for balancing each individual energy storage device. Such system is technically complicated and consequently generates high manufacturing and implementation costs.
Another example of such prior art system is described in US2004/0246635 and illustrated in FIG. 1, FIG. 2a and FIG. 2b, wherein switch series S1 and S2 are switched alternatingly, placing the capacitors (37) and (38) subsequently in parallel to (B1) and (B2), and (B2) and (B3) respectively.
A major disadvantage of this method is its inability to efficiently redistribute charge between non-adjacent energy storage devices in a series connection. All such charge redistribution requires multiple, sequential transfer operations making the process slow and lossy.
Considering the above, it is an object of the present invention to provide a system and method for balancing a series connection of energy storage devices with an improved efficiency and less time consuming balancing, even upon extending the series connection to large numbers of energy storage devices or in case of heavy-duty applications requiring repetitive alternating energy delivery and storage in rapid succession.
Further, it is an object of the present invention to provide a system for balancing a series connection of energy storage devices with a simplified structure while allowing automatically controlled balancing which can be activated and deactivated at any time.
Another object of the present invention is to provide a low-cost system and method for balancing a series connection of energy storage devices, which is easily scalable to large numbers of energy storage devices.
A further object of the present invention is to provide a system and method for balancing a series connection of energy storage devices, which is easily scalable to different types of energy storage devices.
The invention meets the above objects by providing a system and method wherein an intermediate storage element is coupled between a pair of non-adjacent sections of one or a number of adjacent energy storage devices and wherein sequentially is switched between coupling the more positive terminals (A) of the pair of non-adjacent sections to each other via said intermediate storage element and coupling the more negative terminals (B) to each other via said intermediate storage element.